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We used the monochromatic soft-x-ray beamline P04 at the synchrotron-radiation facility PETRA III to resonantly excite the strongest $2p\text{−}3d$transitions in neonlike $\mathrm{Ni}$ions, ${\left[2p^6\right]}_{J=0}\to {\left[{\left(2p^5\right)}_{1/2}3d_{3/2}\right]}_{J=1}$and ${\left[2p^6\right]}_{J=0}\to {\left[{\left(2p^5\right)}_{3/2}3d_{5/2}\right]}_{J=1}$, respectively dubbed $3C$and $3D$, achieving a resolving power of 15 000 and signal-to-background ratio of 30. We obtain their natural linewidths, with an accuracy of better than 10%, as well as the oscillator-strength ratio $f\left(3C\right)/f\left(3D\right)=2.51\left(11\right)$from analysis of the resonant fluorescence spectra. These results agree with those of previous experiments, earlier predictions, and our own advanced calculations. Published by the American Physical Society2024 

Abstract We improve by a factor of 4–20 the energy accuracy of the strongest soft X-ray transitions of Fexviiions by resonantly exciting them in an electron beam ion trap with a monochromatic beam at the P04 beamline of the PETRA III synchrotron facility. By simultaneously tracking instantaneous photon-energy fluctuations with a high-resolution photoelectron spectrometer, we minimize systematic uncertainties down to 10–15 meV, or velocity equivalent ±∼5 km s⁻¹in their rest energies, substantially improving our knowledge of this key astrophysical ion. Our large-scale configuration-interaction computations include more than 4 million relativistic configurations and agree with the experiment at a level without precedent for a 10-electron system. Thereby, theoretical uncertainties for interelectronic correlations become far smaller than those of quantum electrodynamics (QED) corrections. The present QED benchmark strengthens our trust in future calculations of many other complex atomic ions of interest to astrophysics, plasma physics, and the development of optical clocks with highly charged ions.